1. Field of the Invention
The present invention relates to an improved method for producing microcapsules and, more particularly, to an improved method for producing microcapsules having one or more microcapsule walls, wherein at least one of the microcapsule walls is an interpenetrating polymer network formed from the condensation reaction between a functionalized acrylic copolymer and an amine formaldehyde or phenol formaldehyde prepolymer. While the method of the present invention is useful in producing microcapsules generally, microcapsules prepared in accordance with the present invention are particularly useful in carbonless pressure-sensitive recording systems and in photographic photosensitive imaging materials of the type which employ a layer of microcapsules containing a radiation sensitive composition.
2. Description of the Prior Art
Imaging materials employing photosensitive microcapsules are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,846.
In the aforesaid commonly assigned U.S. Patents, images are formed by image-wise exposing a layer of photosensitive capsules to actinic radiation and rupturing the capsules typically by passing the imaging sheet containing the capsules through a pressure nip. The radiation sensitive composition contains a photohardenable or photosoftenable material which undergoes a change in viscosity upon exposure. For example, in the most typical embodiments, the radiation sensitive composition contains a polyethylenically unsaturated monomer which polymerizes upon exposure, thereby causinq the internal phase of the capsules to harden. Due to the difference in the hardness of the capsules in the exposed versus the unexposed areas, only certain capsules rupture and release their contents. If the internal phase contains a dye precursor, the precursor is image-wise released, and a color image is formed upon its transfer to a developer layer. In previously disclosed embodiments, the developer layer may be present on the same support as the layer of capsules or a separate support. It is advantageous if the developer is present on the same support since such a self-contained imaging sheet can be developed as an integral unit.
Microcapsules formulated under the prior art exhibit high sensitivity to moisture and/or humidity. The microcapsule walls tend to absorb moisture and swell.
These swollen microcapsules also contain additional oxygen or are more permeable to oxygen, which is not otherwise present in significant amounts in non-swollen microcapsules. Where the internal phase of the microcapsule contains a photohardenable composition, photohardening is achieved by polymerizing a monomer which is present in the internal phase of the microcapsules, typically by free radical additional polymerization. Where oxygen is present, the oxygen can act as a free radical scavenger, and prevent the radicals from polymerizing the monomer.
Some amount of oxygen is present in all polymeric microcapsules. Thus a finite threshold amount of radiation must be applied to the microcapsules such that the photoinitiator generates a sufficient amount of radicals to deplete the oxygen present. When this threshold level is reached, the radicals generated by the photoinitiator will cause the monomer to polymerize. Oxygen is typically present in concentrations of approximately 10.sup.-3 to 10.sup.-4 moles per liter of internal phase of the microcapsules. This amount must be nearly completely depleted to induce polymerization.
The threshold quantum of radiation necessary to deplete the oxygen present is commonly referred to as the "induction energy" while the light intensity used for exposure is referred to as "induction intensity". The time for which this energy must be applied is referred to as the "induction time".
Induction time varies with the light intensity such that when a higher intensity is applied, a shorter time period is needed to deplete oxygen present and initiate free radical polymerization of the monomer. The reverse is also true. A log-log plot of induction intensity versus induction time yields a reciprocity curve for which, under ideal conditions, the resulting curve is a straight line having a slope of -1.
It has been found that where microcapsules swell due to the presence of moisture, the reciprocity curve suffers severe deviations, particularly in the areas involving lower induction intensities (and correspondingly longer induction times). This deviation is theorized to be caused by the diffusion of oxygen from outside of the capsule walls into the internal phase. Failure is particularly accentuated where the microcapsules are exposed in a relatively high humidity environment, as high humidity significantly deteriorates the barrier properties of the capsule walls.
The result is that under high moisture or humidity conditions using a set induction intensity and time (usually dictated by imaging hardware design parameters), swollen microcapsules fail to polymerize in an image-wise pattern, resulting in inferior reproductions.
Thus, there has arisen a need in the art for a microcapsule having a wall which is less sensitive to humidity.